Python Runtime — Junior¶

Python runs your source as objects and function calls. Start with predictable code before chasing internals.

• Use a virtual environment and record dependencies.
• Know mutability: lists and dicts can change; integers and strings cannot.
• Avoid mutable default arguments; use None and create the list inside.
• Use is None, not == None.

def add_tag(tags=None):
    tags = [] if tags is None else tags
    tags.append("new")
    return tags

Run python -m pytest and reproduce surprising behavior in a tiny script before changing production code.